This invention relates generally to information storage discs and, more particularly, to apparatus for rotating an information disc relative to a radially movable transducer.
Discs for storing large quantities of video information have come into increasing usage in recent years as a result of an increasing need for storage media that provide instantaneous playback, fast random access, and relatively high recording density. The information is typically encoded on the disc in the form of an optically readable sequence of light-reflective and light-scattering regions arranged in substantially circular tracks forming a spiral or concentric circular pattern over the information-bearing surface of the disc.
Ordinarily, the light-reflective and light-scattering regions are initially formed in the disc using an optical transducer for directing onto the disc a collimated beam of high intensity light that is modulated by the information to be recorded. The disc is rotated about its central axis at a substantially constant angular velocity relative to the transducer, while the beam of light is moved radially with respect to the disc at a relatively slow, but constant, velocity. Each revolution of the disc thus results in the production of a seprate, substantially circular information track. When recording video signals, the disc is ordinarily rotated at approximately 1800 r.p.m., whereby each information track contains the information for one video frame.
A typical system for rotating a disc at a constant angular velocity includes an oscillator for producing a reference signal having a prescribed constant frequency, and a servo for locking the angular velocity of the disc onto the frequency of the reference signal. The servo typically includes a spindle motor for rotating the disc, an AC tachometer coupled to the motor for producing a signal having a frequency indicative of the angular velocity of the motor and phase detector for comparing the tachometer signal with the reference signal and producing a control signal proportional to the difference in their respective phase angles. This control signal is suitably processed in a compensation circuit for producing a prescribed frequency response for the servo, and, in turn, amplified and coupled to the spindle motor to appropriately control its angular velocity.
Systems for encoding the discs at a constant angular velocity have not utilized all of the information storing capability of the discs, however, because the successive light-reflective and light-scattering regions forming information tracks near the periphery of disc are significantly larger in size than the corresponding regions forming tracks near the center of the disc. As a result, the density of the recorded information is substantially less at the periphery of the disc than near the center of the disc, and substantially less information can be stored than if the recording density were more uniform.
It will be appreciated from the foregoing that there is a need for a method and apparatus for rotating an information storage disc, relative to a radially movable transducer, at an angular velocity that decreases as the radial position of the transducer increases, whereby information can be recorded on the disc with a more uniform recording density and discs with a substantially longer playing time can be produced. The present invention fulfills this need.